kernel: Rework how DTB/DTBO are built

* Remove the separate dtbo.mk task and merge it
  in kernel.mk
* Merge in-kernel and separated DTBO image generation

Change-Id: I2cae84095cbd3d68abbe1899a9cae34c8a1e5fc9
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index ce86142..97ceca7 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -126,10 +126,8 @@
 endif
 
 # Set DTBO image locations so the build system knows to build them
-ifeq ($(TARGET_NEEDS_DTBOIMAGE),true)
-BOARD_PREBUILT_DTBOIMAGE ?= $(PRODUCT_OUT)/dtbo/arch/$(KERNEL_ARCH)/boot/dtbo.img
-else ifeq ($(BOARD_KERNEL_SEPARATED_DTBO),true)
-BOARD_PREBUILT_DTBOIMAGE ?= $(PRODUCT_OUT)/dtbo-pre.img
+ifeq (true,$(filter true, $(TARGET_NEEDS_DTBOIMAGE) $(BOARD_KERNEL_SEPARATED_DTBO)))
+BOARD_PREBUILT_DTBOIMAGE ?= $(TARGET_OUT_INTERMEDIATES)/DTBO_OBJ/arch/$(KERNEL_ARCH)/boot/dtbo.img
 endif
 
 # Set use the full path to the make command